Field of this Invention
This invention relates to a process and correspondingly designed shaft kilns for the burning of lime, cement or other lumpy bulk materials using coal.
Numerous processes are known in which fuel oil in a fully or partly gasified state or gas is introduced peripherally into a shaft kiln. In addition, kilns have been built that were designed to feed in the fuel--usually gas--centrally from below with the cooling air. These are mainly shaft kilns in which the flow of the kiln charge and the gas are completely or partly countercurrent during the burning process. In this type of kiln the quantity of fuel fed in centrally at the lower end of the shaft is limited by the fact that premature ignition must be avoided in order to prevent hot kiln charge from being discharged. The disadvantage of peripheral fuel feed is that, especially where the cross section of the shaft is large, the penetration depth into the bulk material is unsatisfactory.
Rotary kilns heated by coal dust are also well known. However, the thermal efficiency of this type of kiln is so low that shaft kilns should be used wherever possible. One possibility is the Schmid-Hofer co-current/countercurrent regenerative system (Austrian Patent No. 211,048 and West German Patent (DBP) No. 1,157,133). In such system, two or three shafts are switched alternately to co- and countercurrent, and fuel (gas or fuel oil) is fed only to the respective co-current shaft. The cooling zone is only supplied with air.
At present about 160 kiln systems operate according to such principle. However, they now all run on mineral fuels (oil or gas), of which the price has increased fourfold in recent years. Furthermore, since both supplies and prices of mineral fuels are subject to numerous political influences, further drastic increase can be expected in the next few years.
Another known shaft kiln for the burning of cement, lime and similar materials has a central combustion chamber located at the upper end of the shaft. Such chamber produces heating gases that are no longer combustible as the only source of heat for the burning process [See West German Application (AS) No. 1,205,884 and West German Patent (DBP) No. 1,109,080.]Such type of shaft kiln is intended for use mainly with liquid or gaseous fuels or completely gasified solid fuels.